Argon

Argon is the chemical element. In the periodic table that has the symbol Ar and atomic number 18. The third noble gas, in group 18, argon makes up about 1% of the Earth's atmosphere, making it the most common noble gas on Earth.

Isotopes

The main isotopes of argon found on Earth are Ar-40, Ar-36, and Ar-38. Naturally occurring K-40 with a half-life of 1.250 x 109 years, decays to stable Ar-40 (11.2%) by electron capture and by positron emission, and also transforms to stable Ca-40 (88.8%) via beta decay. These properties and ratios are used to determine the age of rocks.

Related Topics:
Isotope - K - Half-life - Electron capture - Positron emission - Beta decay - Rock

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In the Earth's atmosphere, Ar-39 is made by cosmic ray activity, primarily with Ar-40. In the subsurface environment, it is also produced through neutron capture by K-39 or alpha emission by calcium. Argon-37 is created from the decay of calcium-40 as a result of subsurface nuclear explosions. It has a half-life of 35 days.

Related Topics:
Cosmic ray - Neutron capture - Alpha emission - Calcium - Nuclear explosions

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